As already indicated, in the diseases included here the affection follows certain physiological systems, and we have, in the first place, to inquire whether any explanation of this can be suggested.

General Causation

In almost all the examples of inflammation studied in other parts of this work, the extension of the process is along definite paths. The irritant has been brought to an organ by the blood-vessels, or by some path of transit, such as a mucous canal, .and it has extended, it may be, by the lymphatics or along a surface. In the lesions here to be considered, however, the problem is much more difficult. The localization of the disease bears no relation to the blood-vessels or lymphatic vessels, and if the irritant is brought by the blood there must be local peculiarities leading to differences in the susceptibility of different structures.

In studying the various structures which constitute the cord we saw that in the development of the cord a certain difference is apparent. In the white substance there are certain Fundamental structures constituting the root-zones, and certain Accessory tracts which are separately developed, and mostly of later formation. In the grey substance also there are differences in the periods at which the groups of ganglion cells appear, so much so that some of them which appear later than the rest have been named Accessory nuclei. These are chiefly the median and medio-lateral groups. These various tracts of white substance and groups of ganglion cells have apparently different powers of resistance, and the more recently developed or accessory structures are least resistant. It is not unlikely indeed that in some cases these may be, from their origin, so unstable that without any obvious exciting •cause they may tend to degenerate. In this way the boundary line between actual inflammations and simple degenerations is reached. There are some of the diseases here to be considered which have undoubtedly the characters of inflammation, and even of acute inflammation. But there are others in which the characters are rather those of a chronic degeneration or atrophy of the structures concerned, with very little of an inflammatory character.

There are thus various degrees of susceptibility in the different structures of the cord and medulla oblongata, and when an irritant exists in the blood, developed perhaps in connection with exposure to cold or otherwise, there will be, according to circumstances, various manifestations.